Phase-change materials are Te-containing alloys, typically lying along the GeTe-Sb2Te3 quasibinary tie line. Their ability to switch, reversibly and extremely quickly, between the crystalline and amorphous phases, combined with the high stability of both phases, makes them ideally suitable for memory applications. They have been long used in optical data storage in the form of DVD and Blu-Ray disks and recently have also emerged as a leading candidate for electronic nonvolatile memory devices. In this chapter, a detailed description of these materials is provided starting with the global and local structures of the two phases, which were extensively studied both experimentally and using ab initio computer simulations, and followed by the discussion of possible atomistic mechanisms of the phase-change process, with special accent on the role of electronic excitation. The chapter is concluded by a brief description of the present and emerging applications of this class of chalcogenide materials.